1. Field of the Invention
The present invention is related to the art of machines and methods for profiling of grinding worms.
2. Description of the Related Art
Continuous generating gear grinding with a cylindrical grinding worm has been the most efficient method for many years for the finishing of the toothing of spur or helical gears. The method recently underwent another rapid increase in performance, especially thanks to the high-precision production, that became possible through NC technology, of very complicated kinematic couplings. Not only the increase in productivity which made ever shorter grinding times possible, but also the flexibility of the method and the relatively low tool costs have resulted in grinding machining of toothing taking place increasingly according to the continuous generating gear grinding method.
As regards flexibility, particularly the possibilities that became known recently of grinding topologically modified tooth flanks should be mentioned. Topologically modified tooth flanks refer to, for example, flanks with a crowning over the width and those with a deviation from the involute form, for example with tip reliefs and/or root reliefs, which may be designed differently, also along the tooth width. Geared wheels designed in such a way are used in high-performance gear boxes with the goal of achieving a longer useful life with, at the same time, lower noise emission in all load ranges. The production of such topological tooth flanks requires an accordingly designed grinding worm as well as coordinated process kinematics during grinding. In doing so, a relatively wide grinding worm is used whose thread (or threads) is/are modified differently over the width of the worm. During the machining of the gear wheel, the grinding worm is brought with different areas of its width into contact with the work piece, depending on the work piece's width section just machined. This movement of the grinding worm along its axis as a function of the movement of the work piece along its axis is referred to as "shifting". Particularly the preparation of a topological grinding worm is thus far unfortunately still a time-consuming operation, because not only the pitch of the worm thread may be any desired function of the rotational angle of the worm, but also the profile shape in each axial section may vary over the length of the entire worm thread. Therefore, the desired topology on the tooth flank to be ground must, to a certain extent, first be applied in distorted form onto the grinding worm flank by profiling or dressing, from where, rectified again through the appropriate process kinematics, it is then transferred onto the tooth flank during the grinding process.
In general, a flank 1 of a grinding worm 2 with any desired topology can only be produced with a punctiform-contacting dressing tool 3 which is held by an accordingly controllable device (see FIG. 1) and which is guided line-by-line over the flanks to be dressed. For this purpose, the dressing tool has a toroid work area 4 at its periphery. The dressing procedure can easily be compared with the milling of a forging die: Each individual surface point of the shape to be produced must be machined individually to the proper dimension with the milling cutter--the die-sinking cutter. In this connection, the cutter path over the surface of the shape to be produced typically runs along parallel tracks situated more or less closely to each other. In case of profiling a topological grinding worm, these parallel tracks are situated helix-like on the flanks of the worm profile, that is, on a virtual cylinder around the grinding worm axis.
If simpler shapes of the topology are needed, it is often sufficient to use a profiling tool 6 that machines the flanks 1 on their entire height at one (FIG. 2). In this case, the work area 4 extends over the flanks and the outer perimeter of the tool 6. Of course, only the pitch and the flank angle over the worm width can then be varied, by accordingly controlling the pivoting angle .alpha. and v.sub.ax during the profiling. However, in most cases the required topologies can thereby already be produced.
It is clear that with this simplification, the profiling process becomes considerably quicker than when it takes place line-by-line. A considerable disadvantage of all the aforementioned methods is the fact that the grinding worm cannot be profiled at full rotational speed. The profiling tool must always be moved axially to the worm in the worm thread according to the modulus to be dressed and the rotational speed of the worm; this quickly leads to speeds that can no longer be controlled. The profiling rotational speeds for the grinding worms on today's continuous gear grinding machines are on the order of 100 rpm. That is a rotational speed that is 1/20 to 1/40 the speed needed for grinding. Aside from the resulting relatively long dressing times, a geometry of the worm profile produced however precisely by the profiling process becomes imprecise again at full grinding speed because of deformations due to the centrifugal forces. This understandably becomes all the more important the greater the grinding speed or the work rotational speed of the grinding worm is during grinding. The ideal conditions as they exist on most other grinding machines, namely that profiling takes place at the same grinding wheel rotational speed as grinding takes place, thus cannot be achieved on continuous gear grinders, especially according to the previous method.
In DE-PS 31 34 147, a method is described that does not have these restrictions: a profiling worm rotating synchronously with the grinding worm has the same axial pitch as the worm profile to be dressed and is designed at its active perimeter in such a way that it can dress all occurring worm thread profile shapes. Indeed, this dressing method functions at full rotational speed of the grinding worm, but it has the disadvantage that it cannot be used for topological profiling. The pitch cannot be varied either, because it is predetermined by the dressing worm. In addition, the production of such a dressing worm is very costly and for this reason very high tool costs are incurred.